When drilling oil and gas wells, drill cuttings are produced which must be carried out of the well to surface. This is achieved by entraining the drill cuttings in drilling fluid pumped from surface down a drill string, through a drill bit and returned to surface through the annulus defined between the drill string and the borehole wall.
However, it is often found that in particular during the drilling of deviated or extended reach wells, the flow rate of the fluid returning through the annulus to surface is not sufficient to maintain entrainment of all of the drill cuttings and cuttings may settle in the borehole, restricting well access and increasing the likelihood of other problems, such as differential sticking.
Accordingly, circulating tools have been developed for circulating fluid to facilitate inter alia removal of cuttings. This has been achieved by providing a circulating tool which allows flow of a circulating fluid, typically drilling mud, directly from a string carrying the tool, through flow ports in the tool and into the annulus. This ensures a relatively high flow rate of the drilling mud in the annulus at and above the tool location.
Circulating tools also have further uses. For example, during drilling, some or all of the drilling fluid passing up the annulus can be lost into porous formations, known as loss zones. Such formations may be treated with lost-circulation material (LCM), to prevent or limit further losses. Typically, the LCM is added to the drilling fluid, which is then passed into the annulus via a circulating tool, to plug the formation.
Also, in certain situations, it may be desirable to change the properties of the drilling fluid in the bore—for example, when drilling into high pressure formations, it may be desired to inject relatively high density conditioning mud into a bore. Of course, this requires the existing volume of drilling fluid in the drill string to be circulated to surface. A circulating tool allows circulation of the drilling fluid at a higher flow rate than when, for example, in conventional fluid circulation, fluid is passed through a drilling motor and jetting ports before passing into the annulus and being circulated to surface. Therefore, the circulating tool allows the drilling fluid to be circulated to surface in a shorter time.
One known form of circulation tool includes a body with a flow port which is normally closed by a sleeve, the sleeve also defining a bore-restricting profile. When it is desired to move the sleeve to open the flow port, a plastics ball is inserted into the string at surface and pumped down the string to engage the sleeve profile. This closes the string through bore and the increased fluid pressure above the ball moves the sleeve downwards and opens the flow port.
When it is desired to close the flow port and re-open flow through the tool to the drill bit, a smaller diameter metal ball is pumped down the string, which metal ball closes the flow port and allows elevated fluid pressure above the plastics ball to squeeze the deformable ball through the profile. The metal ball is sufficiently small so as to not to engage the profile, and both balls are then caught by a ball catcher provided below the profile.
Such tools are often unreliable and require components to be discharged down the string. Furthermore, the tools also prevent wireline access through the tool to, for example, Logging While Drilling (LWD) equipment located beneath the circulation tool.
It is amongst the objectives of embodiments of the present invention to provide a circulation tool which obviates or mitigates at least one of the foregoing disadvantages.
It is a further objective of embodiments of the invention to provide a mechanism which may be used to actuate or activate a tool or device, and in particular a downhole tool or device.